Many vacuum capacitors are applied, for example, in the high-frequency power supply circuit used in the semiconductor manufacturing system. When broadly categorizing the vacuum capacitor by its structure, there are two types of vacuum capacitors of a fixed type vacuum capacitor whose capacitance value is fixed and a variable type vacuum capacitor (e.g. Patent Documents 1˜3) whose capacitance value is variable.
As an example of the variable type vacuum capacitor, a capacitor, in which a fixed electrode and a movable electrode are arranged in a vacuum casing and whose capacitance is varied by moving the movable electrode while maintaining a vacuum state in the vacuum casing using bellows etc., is known. As the vacuum casing, an insulation tube body made of insulation material such as ceramic material and seal members made of material of copper etc. are provided, and each opening end side of the insulation tube body is closed by the seal member, then the vacuum casing is formed. Each seal member is formed mainly from a tube member that is provided at the opening end side of the insulation tube body and a cover member that closes the tube member.
The fixed electrode is formed from a plurality of substantially cylindrical electrode members whose diameters are different from each other and which are arranged concentrically (for instance, the cylindrical electrode members are arranged at a certain distance). The fixed electrode is provided at one (hereinafter, called one side seal member, and the other is called the other side seal member) of the seal members inside the vacuum casing. The movable electrode is, same as the fixed electrode, formed from a plurality of substantially cylindrical electrode members whose diameters are different from each other and which are arranged concentrically (for instance, the cylindrical electrode members are arranged at a certain distance). The movable electrode is arranged inside the vacuum casing so that each electrode member of the movable electrode can be inserted into and extracted from a gap between the electrode members of the fixed electrode (the electrode members of the movable electrode are arranged in a staggered configuration so as to be inserted into and extracted from the gap between the electrode members of the fixed electrode and alternately overlap the electrode member of the fixed electrode) with the each electrode member of the movable electrode in noncontact with the electrode members of the fixed electrode. This movable electrode is supported by a movable electrode shaft that moves in an axial direction of the vacuum casing (that moves so as to be able to adjust the extent of the insertion/extraction of the movable electrode with respect to the fixed electrode).
The movable electrode shaft is formed, for instance, from a supporting member (hereinafter, called a movable supporting member) to support the movable electrode and a rod (hereinafter, called a movable rod) that extends from a back surface side of the movable supporting member (e.g. from a surface side of the movable supporting member which faces the other side seal member) to the axial direction of the vacuum casing. This movable electrode shaft is slidably supported (for instance, the movable electrode shaft is slidably supported so that the movable rod can slide in the axial direction of the vacuum casing), for instance, through a bearing member provided at the vacuum casing (e.g. a bearing member fixed at the middle of the cover member).
To adjust the capacitance by moving the movable electrode shaft in the axial direction of the vacuum casing, for example, a member (hereinafter, called a capacitance control unit) that is connected to one end side of the movable rod and rotates by a drive source such as a motor is used. This capacitance control unit is screwed onto the one end side of the movable rod (for example, a female screw part formed at the capacitance control unit is screwed onto the male screw part formed at the one end side of the movable rod), then connects with the movable rod. The capacitance control unit that can rotate by the drive source such as the motor is employed. Further, the capacitance control unit is supported rotatably with respect to the vacuum casing etc. through a supporting member formed from e.g. a thrust bearing.
The bellows is a bellows metal member having expansion/contraction characteristics. The bellows serves as a part of a current path of the vacuum capacitor, and divides the inside of the vacuum casing into a vacuum chamber and an atmospheric chamber. By virtue of this bellows, the movable electrode, the movable supporting member and the movable rod can move in the axial direction of the vacuum casing with a space enclosed by the fixed electrode, the movable electrode and the bellows in the vacuum casing kept airtight as the vacuum chamber (with the space being a vacuum state). For example, one side edge of the bellows is connected to an inner wall side of the other side seal member at the bearing member side, and the other side edge of the bellows is connected to the movable supporting member etc.
Here, with regard to the connection of the bellows, for instance, vacuum brazing is employed. Further, as the bellows, there are some bellows having different structures. For example, bellows having a structure in which the other side edge of the bellows is connected to a surface of the movable rod and bellows having a double bellows structure (e.g. the structure in which stainless bellows and copper bellows are combined) are known.
In the vacuum capacitor having such structure described above, by rotating the capacitance control unit by the drive source such as the motor, rotational motion by the rotation of the capacitance control unit is converted to axial direction motion of the movable electrode shaft, then an overlap area between the fixed electrode and the movable electrode is varied in response to a movement amount of the movable electrode shaft. At this time, the bellows expands or contracts in accordance with the movement of the movable rod.
With this, when voltage is applied to the fixed electrode and the movable electrode and the bellows expands or contracts, the overlap area between the fixed electrode and the movable electrode changes, and a value of capacitance appearing between the both electrodes is seamlessly changed, then the impedance adjustment is made. Regarding high frequency current for the high-frequency apparatus of a case using such vacuum capacitor, the high frequency current flows from the one side seal member to the other side seal member through the bellows and the capacitance between the facing electrodes (between the fixed electrode and the movable electrode).